Quantitative determination of the critical points of Mott metal–insulator transition in strongly correlated systems

Mottness is at the heart of the essential physics in a strongly correlated system as many novel quantum phenomena occur in the metallic phase near the Mott metal–insulator transition. We investigate the Mott transition in a Hubbard model by using the dynamical mean-field theory and introduce the local quantum state fidelity to depict the Mott metal–insulator transition. The local quantum state fidelity provides a convenient approach to determining the critical point of the Mott transition. Additionally, it presents a consistent description of the two distinct forms of the Mott transition points.

Plasmonically induced reflection in metal-insulator-metal waveguides with two silver baffles coupled square ring resonator

A plasmonic waveguide coupled system that is composed of a square ring cavity and a metal-insulator-metal （MIM） waveguide with two silver baffles is proposed. The transmission and reflection properties of the proposed plasmonic system are investigated numerically using the finite element method. The normalized Hz field distributions are calculated to analyze the transmission mode in the plasmonic system. The extreme destructive interference between light mode and dark mode causes plasmonically induced reflection （PIR） window in the transmission spectrum. The PIR window is fitted using the coupled mode theory. The analytical result agrees with the simulation result approximately. In addition, the PIR window can be controlled by adjusting structural parameters and filling different dielectric into the MIM waveguide and the square ring cavity. The results provide a new approach to designing plasmonic devices.

Pressure induced insulator to metal transition in quantum spin liquid candidate NaYbS_(2)

Pressure induced insulator to metal transition followed by the appearance of superconductivity has been observed recently in inorganic quantum spin liquid candidate NaYbSe_(2).In this paper,we study the properties of isostructural compound NaYbS_(2)under pressure.It is found that the resistance of Na YbS_(2)single crystal exhibits an insulating state below 82.9 GPa,but with a drop of more than six orders of magnitude at room temperature.Then a minimum of resistance is observed at about 100.1 GPa and it moves to lower temperature with further compression.Finally,a metallic state in the whole temperature range is observed at about 130.3 GPa accompanied by a non-Fermi liquid behavior below 100 K.The insulator to metal transition,non-monotonic resistance feature and non-Fermi liquid behavior of NaYbS_(2)under pressure are similar to those of NaYbSe_(2),suggesting that these phenomena might be the universal properties in NaLnCh_(2)(Ln=rare earth,Ch=O,S,Se)system.

Design of periodic metal-insulator-metal waveguide back structures for the enhancement of light absorption in thin-film solar cells

To increase the absorption in a thin layer of absorbing material （amorphous silicon, a-Si）, a light trapping design is presented. The designed structure incorporates periodic metal-insulator-metal waveguides to enhance the optical path length of light within the solar cells. The new design can result in broadband optical absorption enhancement not only for transverse magnetic （TM）-polarized light, but also for transverse electric （TE）-polarized light. No plasmonic modes can be excited in TE-polarization, but because of the coupling into the a-Si planar waveguide guiding modes and the diffraction of light by the bottom periodic structures into higher diffraction orders, the total absorption in the active region is also increased. The results from rigorous coupled wave analysis show that the overall optical absorption in the active layer can be greatly enhanced by up to 40%. The designed structures presented in this paper can be integrated with back contact technology to potentially produce high-efficiency thin-film solar cell devices.

Independently tunable dual resonant dip refractive index sensor based on metal–insulator–metal waveguide with Q-shaped resonant cavity

A plasmonic resonator system consisting of a metal–insulator–metal waveguide and a Q-shaped resonant cavity is proposed in this paper. The transmission properties of surface plasmon polaritons in this structure are investigated by using the finite difference in time domain(FDTD) method, and the simulation results contain two resonant dips. The physical mechanism is studied by the multimode interference coupled mode theory(MICMT), and the theoretical results are in highly consistent with the simulation results. Furthermore, the parameters of the Q-shaped cavity can be controlled to adjust the two dips, respectively. The refractive index sensor proposed in this paper, with a sensitivity of 1578 nm/RIU and figure of merit(FOM) of 175, performs better than most of the similar structures. Therefore, the results of the study are instructive for the design and application of high sensitivity nanoscale refractive index sensors.

A layered multifunctional framework based on polyacrylonitrile and MOF derivatives for stable lithium metal anode

Composite Li metal anodes based on three-dimensional(3D) porous frameworks have been considered as an effective material for achieving stable Li metal batteries with high energy density.However,uneven Li deposition behavior still occurs at the top of 3D frameworks owing to the local accumulation of Li ions.To promote uniform Li deposition without top dendrite growth,herein,a layered multifunctional framework based on oxidation-treated polyacrylonitrile(OPAN) and metal-organic framework(MOF) derivatives was proposed for rationally regulating the distribution of Li ions flux,nucleation sites,and electrical conductivity.Profiting from these merits,the OPAN/carbon nano fiber-MOF(CMOF) composite framework demonstrated a reversible Li plating/stripping behavior for 500 cycles with a stable Coulombic efficiency of around 99.0% at the current density of 2 mA/cm~2.Besides,such a Li composite anode exhibited a superior cycle lifespan of over 1300 h under a low polarized voltage of 18 mV in symmetrical cells.When the Li composite anode was paired with LiFePO_(4)(LFP) cathode,the obtained full cell exhibited a stable cycling over 500 cycles.Moreover,the COMSOL Multiphysics simulation was conducted to reveal the effects on homogeneous Li ions distribution derived from the above-mentioned OPAN/CMOF framework and electrical insulation/conduction design.These electrochemical and simulated results shed light on the difficulties of designing stable and safe Li metal anode via optimizing the 3D frameworks.

Rubidium-induced phase transitions among metallic,band-insulating,Mott-insulating phases in 1T-TaS_(2)

Realizing phase transitions via non-thermal sample manipulations is important not only for applications,but also for uncovering the underlying physics.Here,we report on the discovery of two distinct metal–insulator transitions in 1T-TaS_(2) via angle-resolved photoemission spectroscopy and in-situ rubidium deposition.At 205 K,the rubidium deposition drives a normal metal–insulator transition via filling electrons into the conduction band.While at 225 K,however,the rubidium deposition drives a bandwidth-controlled Mott transition as characterized by a rapid collapsing of Mott gap and a loss of spectral weight of the lower Hubbard band.Our result,from a doping-controlled perspective,succeeds in distinguishing the metallic,band-insulating,and Mott-insulating phases of 1T-TaS_(2),manifesting a delicate balance among the electronitineracy,interlayer-coupling and Coulomb repulsion.We also establish an effective method to tune the balance between these interactions,which is useful in seeking exotic electronic phases and designing functional phase-changing devices.

Numerical and experimental evaluation for density-related thermal insulation capability of entangled porous metallic wire material

Entangled porous metallic wire material(EPMWM)has the potential as a thermal insulation material in defence and engineering.In order to optimize its thermophysical properties at the design stage,it is of great significance to reveal the thermal response mechanism of EPMWM based on its complex structural effects.In the present work,virtual manufacturing technology(VMT)was developed to restore the physics-based 3D model of EPMWM.On this basis,the transient thermal analysis is carried out to explore the contact-relevant thermal behavior of EPMWM,and then the spiral unit containing unique structural information are further extracted and counted.In particular,the thermal resistance network is numerically constructed based on the spiral unit through the thermoelectric analogy method to accurately predict the effective thermal conductivity(ETC)of EPMWM.Finally,the thermal diffusivity and specific heat of the samples were obtained by the laser thermal analyzer to calculate the ETC and thermal insulation factor of interest.The results show that the ETC of EPMWM increases with increasing temperature or reducing density under the experimental conditions.The numerical prediction is consistent with the experimental result and the average error is less than 4%.

Impact of the Insulator on the Electric Field and Generation Characteristics of Vacuum Arc Metal Plasmas

The field electron emission plays a vital role in the process of vacuum discharge breakdown. The electric field strength at the cathode tip is significant to the generation char- acteristics of vacuum arc metal plasmas. To increase the field strength at the cathode tip, a coaxial electrode plasma source was employed with an insulator settled between the electrodes. The math expression of the field strength is derived based on the Gauss theory. The impact of the insulator on the electric field and parameters of plasmas were investigated by MAXWELL 3D simulation software and the Langmuir probe. In addition, a composite insulator was adopted to further strengthen the field strength. A series of experiments were performed to focus on the role of the composite insulator in detail. The experimental and simulation results indicate that, a reasonable layout of the insulator, especially the composite insulator, can effectively increase the field strength at the cathode tip and the plasma density.

Hybrid metal-insulator-metal structures on Si nanowires array for surface enhanced Raman scattering

Surface enhanced Raman scattering(SERS)is an efficient technique to detect low concentration molecules.In this work,periodical silicon nanowires(Si NWs)integrated with metal-insulator-metal(MIM)layers are employed as SERS substrates.Laser interference lithography(LIL)combined with reactive ion etching(RIE)is used to fabricate large-area periodic nanostructures,followed by decorating the MIM layers.Compared to MIM disks array on Si surface,the SERS enhancement factor(EF)of the MIM structures on the Si NWs array can be increased up to 5 times,which is attributed to the enhanced electric field at the boundary of the MIM disks.Furthermore,high density of nanoparticles and nanogaps serving as hot spots on sidewall surfaces also contribute to the enhanced SERS signals.Via changing the thickness of the insulator layer,the plasmonic resonance can be tuned,which provides a new localized surface plasmon resonance(LSPR)characteristic for SERS applications.

High density Al2O3/TaN-based metal-insulatormetal capacitors in application to radio equency integrated circuits

Metal-insulator-metal （MIM） capacitors with atomic-layer-deposited Al2O3 dielectric and reactively sputtered TaN electrodes in application to radio frequency integrated circuits have been characterized electrically. The capacitors exhibit a high density of about 6.05 fF/μm^2, a small leakage current of 4.8 × 10^-8 A/cm^2 at 3 V, a high breakdown electric field of 8.61 MV/cm as well as acceptable voltage coefficients of capacitance （VCCs） of 795 ppm/V2 and 268ppm/V at 1 MHz. The observed properties should be attributed to high-quality Al2O3 film and chemically stable TaN electrodes. Further, a logarithmically linear relationship between quadratic VCC and frequency is observed due to the change of relaxation time with carrier mobility in the dielectric. The conduction mechanism in the high field ranges is dominated by the Poole-Frenkel emission, and the leakage current in the low field ranges is likely to be associated with trap-assisted tunnelling. Meanwhile, the Al2O3 dielectric presents charge trapping under low voltage stresses, and defect generation under high voltage stresses, and it has a hard-breakdown performance.

The electrical characteristics of a 4H-silicon carbide metal-insulator-semiconductor structure with Al_2O_3 as the gate dielectric

A 4H-silicon carbide metal-insulator-semiconductor structure with ultra-thin Al2O3 as the gate dielectric, deposited by atomic layer deposition on tile epitaxial layer of a 4H-SiC （0001） 80N-/N＋ substrate, has been fabricated. The experimental results indicate that the prepared ultra-thin Al2O3 gate dielectric exhibits good physical and electrical characteristics, including a high breakdown electrical field of 25 MV/cm, excellent interface properties （1 × 10^14 cm^-2） and low gate-leakage current （IG = 1 × 10^-3 A/cm 2@Eox = 8 MV/cm）. Analysis of the current conduction mecha- nism on the deposited Al2O3 gate dielectric was also systematically performed. The confirmed conduction mechanisms consisted of Fowler-Nordheim （FN） tuaneling, the Frenkel-Poole mechanism, direct tunneling and Schottky emission, and the dominant current conduction mechanism depends on the applied electrical field. When the gate leakage current mechanism is dominated by FN tunneling, the barrier height of SiC/Al2O3 is 1.4 eV, which can meet the requirements of silicon carbide metal-insulator-semiconductor transistor devices.

Structure-dependent metal insulator transition in one-dimensional Hubbard superlattice

We investigate the charge and spin gaps, and the spin structure in half-filled one-dimensional Hubbard superlattices with one repulsive site and L0 free sites per unit cell. For odd L0, it is correlated metal at the particle–hole symmetric point, and then turns into band insulator beyond this point. For even L0, the system has a Mott insulator phase around the particle–hole symmetric point and undergoes a metal–insulator transition with on-site repulsion U increasing. For large U,there exists a multiperiodic spin structure, which results from the ferromagnetic（antiferromagnetic） correlation between the nearest neighboring repulsive sites for odd（even） L0.

Doping dependent metal to insulator transition in the(Bi,Pb)-2212 system:The evolution of structural and electronic properties with europium substitution

The present work investigates the effect of europium substitution on the （Bi, Pb）-2212 system in the concentration range 0.5 ≤ x ≤1.0. Phase analysis and lattice parameter calculations on the powder diffraction data and the elemental analysis of EDX show that the Eu atoms are successfully substituted into the （Bi, Pb）-2212 system. Resistivity measurements （64-300 K） reveal that the system exhibits superconductivity at x ≤ 0.5 and semiconductivity at x 〉 0.5. With the complete suppression of superconductivity which is known to be a quasi-two dimensional phenomenon in these materials, a metal to insulator transition takes place at x = 0.6 and the predominant conduction mechanism is found to be variable range hopping between localized states, resulting in macroscopic semiconducting behaviour. The results of electrical and structural properties of the doped （Bi, Pb）-2212 compounds suggest that the decrease of charge carrier concentration and the induced structural disorder are the more effective and dominant mechanisms in the origin of the metal to insulator transition and suppression of superconductivity due to Eu substitution at its Sr site.

Infrared Nano-Imaging of Electronic Phase across the Metal–Insulator Transition of NdNiO_(3) Films

NdNiO_(3) is a typical correlated material with temperature-driven metal–insulator transition. Resolving the local electronic phase is crucial in understanding the driving mechanism behind the phase transition. Here we present a nano-infrared study of the metal–insulator transition in NdNiO_(3) films by a cryogenic scanning near-field optical microscope. The NdNiO_(3) films undergo a continuous transition without phase coexistence. The nano-infrared signal shows significant temperature dependence and a hysteresis loop. Stripe-like modulation of the optical conductivity is formed in the films and can be attributed to the epitaxial strain. These results provide valuable evidence to understand the coupled electronic and structural transformations in NdNiO_(3) films at the nano-scale.

The Origin of the Giant Hall Effect in Metal-Insulator Composites

Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I composite) can be up to 104 times larger than that in the pure metal called Giant Hall effect. Applying the physical model for alloys with phase separation developed in [1] [2], we conclude that the Giant Hall effect is caused by an electron transfer away from the metallic phase to the insulating phase occupying surface states. These surface states are the reason for the granular structure typical for M-I composites. This electron transfer can be described by [1] [2], provided that long-range diffusion does not happen during film production (n is the electron density in the phase A. uA and uB are the volume fractions of the phase A (metallic phase) and phase B (insulator phase). β is a measure for the average potential difference between the phases A and B). A formula for calculation of R of composites is derived and applied to experimental data of granular Cu1-y(SiO2)y and Ni1-y(SiO2)y films.

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO2-terminated BaTiO3（001） surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO3（001） surface.

In situ electronic structural study of VO_2 thin film across the metal–insulator transition

The in situ valence band photoemission spectrum （PES） and X-ray absorption spectrum （XAS） at V LⅡ-LⅢ edges of the VO2 thin film, which is prepared by pulsed laser deposition, are measured across the metal–insulator transition （MIT） temperature （TMIT=67 ℃）. The spectra show evidence for changes in the electronic structure depending on temperature. Across the TMIT, pure V 3d characteristic d‖ and O 2p-V 3d hybridization characteristic πpd, σpd bands vary in binding energy position and density of state distributions. The XAS reveals a temperature-dependent reversible energy shift at the V LⅢ-edge. The PES and XAS results imply a synergetic energy position shift of occupied valence bands and unoccupied conduction band states across the phase transition. A joint inspection of the PES and XAS results shows that the MIT is not a one-step process, instead it is a process in which a semiconductor phase appears as an intermediate state. The final metallic phase from insulating state is reached through insulator–semiconductor, semiconductor–metal processes, and vice versa. The conventional MIT at around the TMIT=67 ℃ is actually a semiconductor–insulator transformation point.

A SIMPLE SOLUTION FOR MULTILAYER METALLIC OPTICAL WAVEGUIDES

A simple solution for a multilayer metallic optical waveguide by transforming it intoan equivalent three-layer slab waveguide is presented. The dispersion relation of the equivalentthree-layer slab waveguide is solved by using a simple iterative formula. This method itself isexact and can approach any accuracy desired. Moreover, the numerical results for four-layer andfive-layer structures show that the second-order solution is also accurate enough. It is simple andhas the same form of expressions for TE and TM modes and for different layer structures.

Plasmonic Mach-Zehnder interferometric sensor based on a metal-insulator-metal nanostructure

A plasmonic Mach-Zehnder interferometric sensor based on a semicircular aperture-slit nanostructure patterned on a metal-insulator-metal film is proposed and demonstrated by finite difference time domain（FDTD） simulation. Due to the interference between two different surface plasmon polariton modes in this design, the transmission spectra exhibit oscillation behaviors in a broad bandwidth, and can be readily tailored by changing the SPP path length and core layer thickness. Based on this principle, the characteristics of refractive index sensing are also demonstrated by simulation. This structure is illuminated with a collimated light source from the back side to avoid impacts on the interference. Meanwhile,these results show that the proposed structure is promising for portable, efficient, and sensitive biosensing applications.